Richard T. Barber

Synthesis of iron fertilization experiments: From the Iron Age in the Age of Enlightenment

Comparison of eight iron experiments shows that maximum Chl a, the maximum DIC removal, and the overall DIC/Fe efficiency all scale inversely with depth of the wind mixed layer (WML) defining the light environment. Moreover, lateral patch dilution, sea surface irradiance, temperature, and grazing play additional roles. The Southern Ocean experiments were most influenced by very deep WMLs. In contrast, light conditions were most favorable during SEEDS and SERIES as well as during IronEx-2. The two extreme experiments, EisenEx and SEEDS, can be linked via EisenEx bottle incubations with shallower simulated WML depth. Large diatoms always benefit the most from Fe addition, where a remarkably small group of thriving diatom species is dominated by universal response of Pseudo-nitzschia spp. Significant response of these moderate (10–30 μm), medium (30–60 μm), and large (>60 μm) diatoms is consistent with growth physiology determined for single species in natural seawater. The minimum level of “dissolved” Fe (filtrate < 0.2 μm) maintained during an experiment determines the dominant diatom size class. However, this is further complicated by continuous transfer of original truly dissolved reduced Fe(II) into the colloidal pool, which may constitute some 75% of the “dissolved” pool. Depth integration of carbon inventory changes partly compensates the adverse effects of a deep WML due to its greater integration depths, decreasing the differences in responses between the eight experiments. About half of depth-integrated overall primary productivity is reflected in a decrease of DIC. The overall C/Fe efficiency of DIC uptake is DIC/Fe ∼ 5600 for all eight experiments. The increase of particulate organic carbon is about a quarter of the primary production, suggesting food web losses for the other three quarters. Replenishment of DIC by air/sea exchange tends to be a minor few percent of primary CO2 fixation but will continue well after observations have stopped. Export of carbon into deeper waters is difficult to assess and is until now firmly proven and quite modest in only two experiments.

Biological Consequences of El Niño

Observations of the 1982-1983 El Ni�o make it possible to relate the anomalous ocean conditions to specific biological responses. In October 1982 upwelling ecosystems in the eastern equatorial Pacific began a series of transitions from the normal highly productive condition to greatly reduced productivity. The highly productive condition had returned by July 1983. Nutrients, phytoplankton biomass, and primary productivity are clearly regulated by the physical changes of El Ni�o. Evidence from 1982 and 1983 also suggests effects on higher organisms such as fish, seabirds, and marine mammals, but several more years of observation are required to accurately determine the magnitude of the consequences on these higher trophic levels.

An estimate of new production in the equatorial Pacific

Abstract Two estimates of new production in the equatorial Pacific are presented and the similarity between the physically based estimate and an estimate based on measurements of primary productivity is emphasized. The new production calculated by both methods is on the order of 1 gigaton C y−1 (1 × 1015 g C y−1), an order of magnitude greater than previous estimates for this region. The new estimates suggest that equatorial Pacific circulation supports a significant proportion (18–56%) of the global new production calculated by Eppley and Peterson (1979, Nature, 282, 677–680).

African dust and the demise of Caribbean coral reefs.

The vitality of Caribbean coral reefs has undergone a continual state of decline since the late 1970s, a period of time coincidental with large increases in transatlantic dust transport. It is proposed that the hundreds of millions of tons/year of soil dust that have been crossing the Atlantic during the last 25 years could be a significant contributor to coral reef decline and may be affecting other ecosystems. Benchmark events, such as near synchronous Caribbean-wide mortalities of acroporid corals and the urchin Diadema in 1983, and coral bleaching beginning in 1987, correlate with the years of maximum dust flux into the Caribbean. Besides crustal elements, in particular Fe, Si, and aluminosilicate clays, the dust can serve as a substrate for numerous species of viable spores, especially the soil fungus Aspergillus. Aspergillus sydowii, the cause of an ongoing Caribbean-wide seafan disease, has been cultured from Caribbean air samples and used to inoculate sea fans.

Physical and biological controls on carbon cycling in the equatorial pacific.

The equatorial Pacific is the largest oceanic source of carbon dioxide to the atmosphere and has been proposed to be a major site of organic carbon export to the deep sea. Study of the chemistry and biology of this area from 170� to 95�W suggests that variability of remote winds in the western Pacific and tropical instability waves are the major factors controlling chemical and biological variability. The reason is that most of the biological production is based on recycled nutrients; only a few of the nutrients transported to the surface by upwelling are taken up by photosynthesis. Biological cycling within the euphotic zone is efficient, and the export of carbon fixed by photosynthesis is small. The fluxes of carbon dioxide to the atmosphere and particulate organic carbon to the deep sea were about 0.3 gigatons per year, and the production of dissolved organic carbon was about three times as large. The data establish El Ni�o events as the main source of interannual variability.

Primary productivity and its regulation in the equatorial Pacific during and following the 1991–1992 El Niño

The cycling of carbon in the equatorial Pacific Ocean was investigated by the Equatorial Pacific (EqPac) Study in 1992. As part of that study in situ primary productivity was measured on survey and time-series cruises along 140°W from 12°N to 12°S with methods determined to be trace-metal clean. Primary productivity, chlorophyll and chlorophyll-specific productivity rates varied coherently in relation to two large-scale features: temporally, primary productivity was reduced during the El Nino dominated period (February–April 1992) and increased during the cool period (August–October 1992); and spatially enhanced primary productivity persisted close to the equator relative to the oligotrophic regions poleward of 10°N and 10°S. On the equator in October 1992 during the period of relatively cool water, primary productivity was about twice (125 mmol C m−2 day−1) the value during the peak warm period (60 mmol C m−2 day−1). The climatological mean equatorial productivity in the cold tongue has been recalculated to be about 75 mmol C m−2 day−1 (Chavez et al., 1996). The mean 1992 productivity on the equator (1°S–1°N) was about 25% higher than climatology (95 vs 75 mmol Cm−2 day−1) and about 3 times the value in oligotrophic waters poleward of 10°N and 10°S (95 vs 30 mmol C m−2 day−1). Higher chlorophyll-specific productivity during the cool period relative to the warm period (3.9 vs 2.4 mmol C mg chl−1 day−1) indicates that the increase in absolute productivity did not result solely from a biomass increase, but from a change in the nutrient-regulated specific productivity rate. The regulating nutrient was not a macronutrient, such as nitrate or silicic acid, because macronutrients (and light) were present in uptake-saturating concentrations during both the warm and cool periods of the 1992 EqPac study. Physiological constraint by a micronutrient, such as iron, is implicated as the factor regulating these productivity variations. The change in iron supply resulted from a change in equatorial circulation processes. During the warm period, El Nino-Southern Oscillation (ENSO)-driven changes in pycnocline topography depressed the Equatorial Undercurrent (EUC), thereby decreasing the amount of iron-rich EUC water entrained into equatorial upwelling and vice versa during the cool period. During the August–October cool period of generally increased productivity, two further episodic increases in specific productivity, biomass and diatom abundance occurred during intense and remotely forced upwelling events associated with a front or the passage of a frontal wave. In both mesoscale episodes, temperature and salinity show that the intensified upwelling reached more deeply into the already relatively shallow EUC. Productivity and biomass increases during both of these events were quantitatively similar to those in an in situ iron addition experiment (IronEx) carried out in equatorial Pacific waters in 1993. Variations in the supply of upwelled iron provided by the iron-rich EUC best account for the warm-cool period difference in phytoplankton productivity as well as the episodic increases in specific productivity, biomass and diatom abundance during intense mesoscale upwelling events seen in the dynamic equatorial region in the EqPac study.

Photosynthetic characteristics and estimated growth rates indicate grazing is the proximate control of primary production in the equatorial Pacific

Macronutrients persist in the surface layer of the equatorial Pacific Ocean because the production of phytoplankton is limited; the nature of this limitation has yet to be resolved. Measurements of photosynthesis as a function of irradiance (P-I) provide information on the control of primary productivity, a question of great biogeochemical importance. Accordingly, P-I was measured in the equatorial Pacific along 150°W, during February-March 1988. Diel variability of P-I showed a pattern consistent with nocturnal vertical mixing in the upper 20 m followed by diurnal stratification, causing photoinhibition near the surface at midday. Otherwise, the distribution of photosynthetic parameters with depth and the stability of P-I during simulated in situ incubations over 2 days demonstrated that photoadaptation was nearly complete at the time of sampling: photoadaptation had not been effectively countered by upwelling or vertical mixing. Measurements of P-I and chlorophyll during manipulations of trace elements showed that simple precautions to minimize contamination were sufficient to obtain valid rate measurements and that the specific growth rates of phytoplankton were fairly high in situ, a minimum of 0.6 d−1. Diel variability of beam attenuation also indicated high specific growth rates of phytoplankton and a strong coupling of production with grazing. It appears that grazing is the proximate control on the standing crop of phytoplankton. Nonetheless, the supply of a trace nutrient such as iron might ultimately regulate productivity by influencing species composition and food-web structure.

Phytoplankton variability in the central and eastern tropical Pacific

An extensive set of measurements of phytoplankton production, biomass and composition, and microzooplankton grazing from the coast of Peru to 170°W during 1992, together with similar data collected over the previous decade, has allowed recalculation of the primary production supported by equatorial upwelling and improved description of the variability in phytoplankton properties. Equatorial region surface chlorophyll and phytoplankton biomass were low, averaging 0.2 μg 1−1 and 20 μg C 1−1, respectively, and showed low variance. Phytoplankton in the open ocean of the tropical Pacific were dominated by small < 5 μm) solitary organisms, primarily prochlorophytes, Synechococcus, eukaryotic picoplankton, haptophytes and dinoflagellates, while coastal populations were dominated by larger organisms or colonies (primarily diatoms). At a few open ocean locations high numbers of diatoms were found. The chlorophyll maximum observed in the equatorial Pacific was a function of increased chlorophyll per cell rather than an increase in cell numbers. Surface phytoplankton carbon to chlorophyll was highly variable and a function of available irradiance and upwelling strength. On the order of 40% of the particulate nitrogen retained by GF/F filters was estimated to be phytoplankton nitrogen. Phytoplankton growth rate estimates using daily carbon uptake and phytoplankton carbon estimated from microscopic enumeration ranged from 0.55 to 0.70 day−1. Estimates of growth rates from dilution experiments gave estimates of the order of 1 day−1 and microzooplankton grazing rates that were significantly lower, 0.4 day−1. The mean mass specific grazing rate for microzooplankton was estimated to range from 1.6 to 1.8 day−1. The mean productivity for the equatorial Pacific from 90° to 180°W, 5°N−5°S, was estimated to be 900 mg C m−2 day−1 for the period from 1990 to the present, twice that estimated previously. The maximum f-ratio (new to total production) was estimated to be 0.36. Assuming that between 25 and 50% of the upwelled nitrate is never taken up by phytoplankton between 5°N and 5°S, new production would be 162–244 mg C m−2 day−1 and f would range from 0.18 to 0.27.

Primary production off northwest Africa: the relationship to wind and nutrient conditions

Abstract The flux of carbon in a eutrophic coastal upwelling region was studied in the Cap Blanc area off northwest Africa from March through May 1974. Primary production was consistently between 1 and 3 g C m −2 day −1 except inshore, where turbidity limited light penetration. Productivity reached a maximum in the mid-shelf zone and then decreased as the water moved offshore and nutrient levels declined. Chlorophyll a averaged 63 mg m −2 and was distributed similarly to carbon fixation. Assimilation numbers were inversely related to the intensity of the northeast trade wind. Photosynthesis was depressed during periods of deep mixing because the phytoplankton were maintained at an average depth corresponding to about the 10% light level. The data indicate that mixing can limit primary productivity on a time scale of days and can make the difference between a moderately productive and highly productive upwelling system.

New production along 140°W in the equatorial Pacific during and following the 1992 El Niño event

Abstract This study was conducted as part of two JGOFS transects along 140°W between 12°N and 12°S during February–March 1992 and August–September 1992. Although its purpose was to investigate seasonal variability in nitrogenous nutrient availability and biological utilization in support of primary production, the occurrence of the 1992 El Nino during the first transect permitted us to compare El Nino and post-El Nino conditions. We had hypothesized that an El Nino-related reduction in upwelling of cold nutrient-rich water would lead to a reduction in surface nutrient concentrations and rates of new and primary production in the vicinity of the equator. However, during the height of the El Nino, NO3− concentrations from 2°N to 7°S remained high enough (> 2 μmol kg−1) to preclude nitrogen-limited primary production. Total nitrogen uptake rates measured 6 months after the El Nino were 2.4 times greater than those observed during the El Nino. On both transects, mean values for NH4+ uptake rates were 8 times those for NO3− uptake. Mean rates of new production integrated to the 1% light depth over the full transects were 4.3 mmol C m−2 day−1 during the El Nino, and 9.9 mmol C m−2 day−1 6 months later. Within the 2°N-2°S region, rates of new production were 4.8 and 18.5 mmol C m−2 day−1 for the first and second transects, respectively. Ratios of carbon fixed in primary production and nitrogen uptake averaged 7.7 and 5.1 (mole ratio) for the transects during and after the El Nino, respectively. Even though both the rates of primary production and NO3− concentrations were higher after the El Nino, there was a strong suppressing effect of NH4+ concentration on NO3− uptake. On both transects local minima in f-ratios (0.06) were evident within 1° of the equator. The mean f-ratio for 2°N-2°S was slightly lower and less variable (0.06-0.13; x =0.11 ) during the El Nino than after (0.08-0.20; x=0.13). Over a broader meridional band (5–7°N to 5–8°S) f-ratios during the El Nino were similar to values determined in 1988, a non-El Nino year, during the same season. Diel periodicity was evident in NO3− uptake between 2°N and 3°S, reaching 10- and five-fold day vs night enhancement during and after the El Nino, respectively. Following the El Nino, the diel cycle in NO3− uptake was strongly skewed to the early portion of the light day in the most NO3−-rich waters. These and other comparisons between the two transects serve to indicate that phytoplankton species assemblages and/or nutritional sufficiency of micro-nutrients were different during and after the El Nino. On both transects plankton nutritional preferences resulted in nitrate-sparing conditions in the vicinity of the equator. In spite of high primary productivity, f-ratio calculations and turnover times for NH4+ suggest that local rates of remineralization were sufficient to meet 87–90% of the nitrogen demand in the 2°N-2°S region, resulting in residence times for NO3− of 305 days during the El Nino and 190 days 6 months later. A potential implication of this condition is a correspondingly low export of the particulate product of photosynthesis to the deep ocean. Water column density structure and nutrient distributions argue for reduced rates of nutrient upwelling during the El Nino event. Altered upper-ocean physics and concomitant changes in plankton community structure and function allowed for more extensive upper-ocean nutrient recycling, and presumably reduced export, of primary production during the El Nino. As a consequence the depletion time of recently upwelled NO3− remained long, and thus this nutrient was conserved during the period of diminished supply from upwelling. While these patterns imply direct regulation of new production by the availability of NH4+, the role of a micro-nutrient such as Fe that influences (1) the species composition of the phytoplankton assemblage, and associated potential for export from rather than recycling within the euphotic zone or (2) the sensitivity of NO3− uptake to NH4+ presence, cannot at this time be properly evaluated. Significantly higher rates of new production with only a small increase in f-ratio in the period following the El Nino may constitute a more prominent feature in the ENSO cycle of equatorial biological production and export than the El Nino event per se. Whether this is a general feature in the ENSO cycle, or unique to the period of our study, which was one of unusual global atmospheric conditions, has yet to be established.